The present invention relates to a method of electrophotographic image formation and in particular to a method of electrophotographic image formation according to which images of high quality can be always stable reproduction with desirable density.
Hitherto, there have been well known so-called CPC system which comprises carrying out charging and exposing to form a static latent image on photoreceptor and then developing the latent image to directly form a toner image on the photoreceptor and so-called PPC system by which the toner image formed on photoreceptor by development is transferred onto a plain paper.
Furthermore, in the field of printing in which a half-tone film is made by color separation of original and this is used as a printing plate or information obtained by color separation of original is converted to digital signal, which is directly used as a printing plate, usually various quality tests (control) are conducted using a proof print which approximates to print before printing and it has been studied to utilize rapid and inexpensive electrophotographic process for this proof.
In such electrophotographic process, since electric charge of static latent image subtlely changes depending on environmental conditions such as temperature or humidity, color reproducibility is influenced by the environmental conditions even under the same setting conditions, therefore so adjustment by skilled workers in each necessary occasion and by complete air conditioning, or adjustment must be made by complicated control device. Thus, operation, device or apparatus become complicated.
Moreover, in the field of printing, the print must be the same as or approximate with appearance of commercial articles rather than be faithful to original image and there is the tendency to attach importance to artistic property and severer accuracy is demanded also in balance of density. Therefore, in this field, in order to make it more close to print, various efforts have been made in an attempt to satisfy the severe conditions, for example, method of control of quality called ink proofing has been employed for a long time, and thus time has been spent or expensive materials or apparatuses have been used.
For obtaining images in good balance without fogging of image density or insufficient density by electrophotographic process, a method has been known as disclosed in Japanese Patent Kokai No. 63-149659.
The method disclosed in Japanese Patent Kokai No. 63-149659 is a method for formation of color images by subjecting a photoreceptor to a step including charging, exposing the photoreceptor to form an electrostatic latent image and development which is repeated a plurality of times, characterized in that charging conditions are set for every time of the step so that difference between surface potential of photoreceptor and developing bias potential at respective development positions by respective developing machines used for development becomes substantially constant.
However, since according to the above method, charging conditions are set and controlled for every step comprising charging, forming of static image and developing so that difference between surface potential of photoreceptor and bias potential becomes constant, desired control cannot be easily performed owing to influence of environmental conditions from charging to developing or charging conditions must be set and controlled with also sensing the change of environmental conditions and considering change of surface potential of photoreceptor caused by the change of environmental conditions. Thus, because of complication of operability and apparatus, there are still many points to be improved. Furthermore, since according to the above method, charging conditions are set and controlled for every one step, this method cannot be applied to monochromatic system and besides, since charging conditions are controlled, it is not easy to set maximum image density for every color.
Under the circumstances, there is strongly demanded a method which utilizes electrophotographic process and is rapid, inexpensive and simple.
As a result of research on electrophotographic process conducted by the inventors for a long time, it has been found that the maximum image density can be maintained constant by measuring surface potential of maximum image density portion just before developing and setting developing bias potential based on the measured surface potential so that difference between the surface potential and developing bias potential is constant. Thus, the present invention has been accomplished.